1. Field
The present disclosure relates generally to spray methods for delivering saccharification enzymes and fermentation organisms onto lignocellulosic biomass. More specifically it relates to spray methods using a pretreatment prehydrolysate as the delivery liquid in hydrolysis or simultaneous hydrolysis and fermentation processes, thereby reducing the amount of dilution and the need to mix the saccharification or fermentation reaction mixture, while increasing the titer of a saccharification and/or fermentation product.
2. Related Art
In the hydrolysis and fermentation of cellulosic biomass to ethanol or other biofuels and bioproducts, the titer of the final product is lower than starch based fermentation, and therefore requires more energy for distillation. A minimum solids loading of 10% going into hydrolysis has been suggested as having an acceptable level of distillation energy requirements. M. Galbe & G. Zacchi, A review of the production of ethanol from softwood, Applied Microbiology and Biotechnology 59, 618-628 (2002).
Increasing final product titer may be achieved with higher solids loadings, but handling high solids introduces mixing requirements that increase the complexity and energy cost of the process. Mixing power requirements increase with increasing solids loadings, so non-conventional mixing techniques have been applied to biomass reactions at high solids concentration. For example, a free-fall mixer has been applied to wheat straw loadings of up to 40% solids. Henning Jorgensen, et al., Liquifaction of lignocelluloses at high solids concentrations, Biotechnology and Bioengineering 96, 862-870 (2006). Mixing may be accomplished in a number of other ways, including high consistency mixers, screw presses, static mixers, or by spray application. One example of spray application of acid is Yan et al. L. Yan, et al., Dilute sulfuric acid cycle spray flow-through pretreatment of corn stover for enhancement of sugar recovery, Bioresource Technology 100, 1803-1808 (2009). High solids may also slow hydrolysis in the reactor for reasons other than mixing limitations. Jan B. Kristensen, et al., Yield-determining factors in high solids enzymatic hydrolysis of lignocellulose, Biotechnology for Biofuels 2, 11 (2009).
Product titer may also be increased by reducing dilution steps in hydrolysis and fermentation processes. Methods of lignocellulose hydrolysis and/or fermentation include separate hydrolysis and fermentation (SHF—hydrolysis first, then fermentation) or simultaneous saccharification and fermentation of lignocelluloses (SSF—hydrolysis and fermentation happens at the same time). Saccharification and fermentation of lignocellulosic biomass typically require an acidic and/or a chemical pretreatment process or an alkaline chemical pretreatment process. In a biomass pretreatment process without complete biomass hydrolysis, a pretreated biomass solids stream and a prehydrolysate stream are generated. The pretreated biomass stream (solids fraction) is rich in cellulose while the prehydrolysate stream (liquid fraction) is rich in hemicellulose sugars or hemicellulose oligomers, along with lignin, extractives, furans, aldehydes, acetic acid, or other inhibitors that restrict the growth and productivity of a fermenting organism. Additionally, the prehydrolysate stream usually has a pH outside of the typical enzymatic hydrolysis pH range (4.8 to 6.5) or typical fermentation pH range (3.5 to 7.5). Similarly, the pretreated biomass solids also may also contain inhibitors and a different pH from the enzymatic hydrolysis pH and the fermentation pH. Therefore, prehydrolysate and pretreated biomass conditioning are often needed before an enzymatic hydrolysis and a fermentation process.
The prehydrolysate is typically separated from the solids after pretreatment and conditioned by overliming (Andy Aden, Biochemical Production of Ethanol from Corn Stover: 2007 State of Technology Model, NREL/TP-510-43205 (2008)) or ion exchange (J. Y. Zhu, et al., Ethanol production from SPORL-pretreated lodgepole pine: preliminary evaluation of mass balance and process energy efficiency, Applied Microbiology and Biotechnology 86, 1355-1365 (2010)) and fermented separately from the solids fraction. In these scenarios, it is necessary to add dilution to accomplish pH changes, enzyme loading, and, in the case of simultaneous saccharification and fermentation (SSF), fermentation organism loading.
The solids fraction of the pretreatment process is typically washed to remove fermentation inhibitors, then hydrolyzed, and fermented either separately or together with the prehydrolysate. Since both the prehydrolysate and the hydrolysate contain sugars that could be fermented to maximize yield, any water added up to the point of fermentation reduces the ultimate product titer. This requires greater energy requirements for separation with distillation. The high initial viscosity and associated poor mixing properties of the solids fraction of the pretreated cellulosic biomass may also reduce the product titer. For lab scale experimentation, pretreated cellulose is often hydrolyzed at 5% solids or even lower to enable mixing, and mixing effectiveness decreases (or requires more power) as solids increase. The difference between 10% solids and 20% solids going into hydrolysis, for example, is a factor of two on the ultimate product titer, resulting in approximately a factor of two in distillation energy use. G. Zacchi & A. Axelsson, Economic evaluation of preconcentration in production of ethanol from dilute sugar solutions, Biotechnology and Bioengineering 34, 223-233 (1989). Greater hydrolysis reactor volume, fermentor volume and distillation column volume will be required for lower product titer liquid handling, potentially tripling the total capital cost of downstream processing.
In both SHF and SSF, it has been anticipated that “to increase the sugar concentration in a future large-scale operation, it is assumed that the whole slurry after pretreatment would be used without introducing separation steps that would dilute the process stream.” M. Galbe & G. Zacchi, Applied Microbiology and Biotechnology 59, 618-628 (2002). Therefore, what is needed is a method of hydrolyzing and fermenting biomass compositions to produce product liquids of high titer with minimal dilution steps. The method would make use of the prehydrolysate to the extent needed to optimize mixing, enzymatic hydrolysis, and fermentation. The prehydrolysate stream may be used to accomplish pH changes, enzyme loading, and, in the case of SSF, fermentation organism loading to avoid adding dilution water.